(1) Field of the Invention
The invention relates to a method of measuring torque acting on a drive shaft and a torque measurement system for implementing said method. Although the method according to the invention and the measurement system according to the invention can fundamentally be used for a plurality of different applications, their use as a drive shaft torque measurement system, i.e., as a system for determining the torque or rotational moments acting on the tail rotor drive shaft of a helicopter, is presented in detail below. The term “tail gearbox” is to be understood as well as “intermediate gearbox”.
(2) Description of the Prior Art
A tail rotor drive shaft between a main gearbox and a tail gearbox of a helicopter is submitted to varying torsional moments depending from the specific flight operation of said helicopter. It is interesting to know the amount and direction of any such torsional moment preferably at any moment.
The torsional moments at a drive shaft in flight operation may be monitored with a measurement system with strain sensors bonded to the drive shaft and signal transmission via a telemetry unit. Such a measurement system has the disadvantages of strain gauges with a limited lifetime, a low reliability of bonding and high additional weight of balance mass and telemetry. For retrofit of said measurement system structural modifications of the helicopter are required with the implied high costs. The telemetry system of said measurement system may cause electromagnetic interference with adjacent control units and thus may be at the origin of hazards and disturbances for the flight operation of such a helicopter.
The document U.S. Pat. No. 5,067,355 (A) discloses a method and an apparatus for determining the power output of a gas turbine engine utilizing a speed sensor coupled to the engine shaft and a signal developed by a pyrometer coupled to the engine turbine for determining engine temperature. A signal from the speed sensor derived from a toothed wheel passing adjacent the sensor is processed to obtain a train of pulse signals representative of angular rotation of the engine shaft. The pyrometer is mounted adjacent at least one of the turbine disks of the engine for detecting instantaneous temperature of each blade of the turbine disk as it passes by the pyrometer. The signal developed by the pyrometer is therefore a signal having peaks corresponding to passage of each blade at the pyrometer. A signal conditioning apparatus processes the signal from the pyrometer to develop a pulse train of shaped signals corresponding to the angular position of each turbine blade as it passes by the pyrometer. The system determines the relative phase difference between the signals developed by the speed sensor and the signals developed on the pyrometer under low load conditions and stores this information as a reference phase difference value. Phase differences under load conditions are thereafter compared to the reference value. The differences in phase are proportional to shaft twist and accordingly to shaft torque. Said method and apparatus are suitable only for a drive shaft with rotating blades.
The document U.S. Pat. No. 5,265,480 (A) discloses first and second magnetic layers respectively fixed in two positions on a peripheral face of a rotary shaft. A magnetizing pattern is preformed at a constant pitch solely on a magnetic face of the first magnetic layer. In a preparatory step, the magnetizing pattern is copied onto a magnetic face of the second magnetic layer, so that a torque detection step is started. In the torque detection step, there is detected phase difference between reproducing signals of two magnetic heads owing to the torsional deformation of the rotary shaft caused by torque. In addition, the number of rotations for a unit time of the rotary shaft is detected based on one of the reproducing signals. Consequently, the applied torque can be obtained by calculation based on the phase difference and the number of rotations for a unit time. The arrangement of specific magnetic layers is not always possible.
The document U.S. Pat. No. 3,538,762 (A) discloses a rotary shaft torque measuring system with a first toothed wheel attached directly to the rotary shaft, a second toothed wheel and transducer means mounted adjacent said toothed wheels for generating alternating signals in response to the rotation of said wheels having a phase displacement indicative of torsional deflection of said shaft. Said rotary shaft torque measuring system is complicated and bulky.